The last suggested exit node needs to be incorporated in the decision
making process when a new suggestion is requested, but currently it is
not quite right: it'll be used if the suggestion code has an error or a
netmap is unavailable, but it won't be used otherwise.
Instead, this makes the last suggestion into a tiebreaker when making a
random selection between equally-good options. If the last suggestion
does not make it to the final selection pool, then a different
suggestion will be made.
Since LocalBackend.SuggestExitNode is back to being a thin shim that
sets up the parameters to suggestExitNode, it no longer needs a test.
Its test was unable to be comprehensive anyway as the code being tested
contains an uncontrolled random number generator.
Updates tailscale/corp#19681
Change-Id: I94ecc9a0d1b622de3df4ef90523f1d3e67b4bfba
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
0219317372